1. Field of the Invention
This invention generally relates to a developing device. More particularly, it relates to a developing device in which a uniform thin layer of one-component developer is formed on developer holding means and this layer is opposed to a latent image bearing member to develop the same.
2. Description of the Prior Art
For the developing devices using one-component developer, various methods are known or have been proposed.
Among these, the jumping developing method is known as a unique method. This method comprises applying developer as a thin layer onto developer holding means, thereafter opposing an electrostatic latent image surface to the surface of the thin layer of developer with a small gap interposed therebetween, and then causing the developer to jump from the developer holding means to the electrostatic latent image surface by the electrostatic attraction thereof to thereby effect development (see U.S. Pat. Nos. 2,839,400 and 3,232,190). According to this method, the developer is not only not attracted to the non-image bearing portion having no latent image potential but also does not contact the non-image bearing portion and therefore, good development free of fog can be accomplished. Further, where use is made of one-component developer using no carrier particle, there are also obtained good effects that the mixture ratio of the developer does not fluctuate and that there is no deterioration of carrier particles.
Also, the Applicant has proposed, as developing methods different from this jumping developing method, entirely novel developing methods as disclosed in U.S. Patent Applications Ser. No. 938,101, filed Aug. 30, 1978, abandoned; Ser. No. 938,494, filed Aug. 31, 1978, abandoned; Ser. No. 058,434, filed July 18, 1979, abandoned; and Ser. No. 058,435, filed July 18, 1979, U.S. Pat. No. 4,292,387.
The former two developing methods comprise disposing one-component magnetic developer, developer holding means (non-magnetic) and magnetic field generating means in the named order, forming a uniform thin layer of developer on the developer holding means by the magnetic force of the magnetic field generating means, and opposing the surface of the thin layer of developer to the electrostatic latent image forming surface with a minute gap interposed therebetween so that the former surface does not contact the latter surface. The developer opposed to the image bearing portion is extended by the electrostatic attraction thereof to thereby effect development. These methods can also obtain developed images free of fog because development is effected without the developer contacting the non-image bearing portion.
The latter two methods comprise disposing one-component magnetic developer, developer holding means (non-magnetic) and magnetic field generating means in the named order, forming a uniform thin layer of developer on the developer holding means by the magnetic force of the magnetic field generating means, and opposing the surface of the thin layer of developer to the electrostatic latent image forming surface with a minute gap interposed therebetween so that the former surface does not contact the latter surface. As a developing bias voltage, an AC bias voltage is applied to the same and further, the gap between the electrostatic latent image surface and the developer holding means is varied with time to thereby effect development. By these latter methods, in the initial stage of the development, the developer is caused to arrive at the non-image bearing portion of the electrostatic latent image as well to thereby effect the development of the half-tone portion and the developer is caused to arrive only at the image bearing portion with time to thereby effect the development of such portion. By this, there is obtained the effect of development which is excellent in half-tone reproducibility as compared with the former methods and moreover is free of fog.
According to such developing methods in which development is effected with the thin layer of one-component developer disposed in opposed relationship with the latent image surface, there are obtained very excellent effects in respect of developing performance, image reproducibility, life of developer, etc. as compared with the conventional methods. However, even these developing methods may in some cases encounter the following problems when actually put into practice.
(1) By the developing action, the history of development is left on the layer of developer on the developer holding means to cause a ghost image to be developed during the next cycle of development.
In the aforementioned developing methods, the developer layer formed on the developer holding means is a very thin layer. Therefore, once this developer layer is used for development, a great difference in thickness of the developer layer is created between the portion thereof corresponding to the image bearing portion and the portion thereof corresponding to the non-image bearing portion. Such thickness history of the developer layer is intactly left even if fresh developer is supplied after the developer layer has been used for development, to greatly affect the next cycle of development and cause a disadvantage that the so-called ghost which is a negative pattern of the previously developed image is created in the next cycle of development. This is considered to be attributable to the inability to sufficiently replenish the surface of the developer holding means from which the developer has been brought away with fresh developer in the developer supplying step thereafter (or to some difference in characteristic (especially, tribo property) between the developer remaining on the surface of the developer holding means and the developer newly supplied thereto).
(2) A thin layer of developer is formed on the surface of the developer holding means in a long time during which the developing action is carried out, to reduce the developing performance.
During the developing action, the developer always repeats contact with and separation from the surface of the developer holding means, so that the surface of the developer holding means is contaminated by the developer and a film layer of developer is liable to form thereon. This film layer consists of resin component of low molecular weight contained in the developer, or a control agent for providing the tribo property, or a layer of very minute powder which does not contribute to the development. In any case, if these components cover the surface of the developer holding means, the amount of charge of the developer becomes deficient particularly in the case of the developing methods utilizing the charge created by the friction between the developer and the developer holding means, thus reducing the developing density and/or aggravating the image reproducibility.
(3) In a long time during which the developing action is carried out, aggregation of the developer is created or the adhesive force between the developer and the surface of the developer holding means is enhanced to make difficult the formation of a layer having a uniform thickness and cause irregularity of development.
There is no problem when the developer applied onto the developer holding means contributes to development and separates from the surface of the developer holding means within a short time, but if developer which does not contribute to development because of its low developing density is left on the surface of the developer holding means for a long time, the aggregation force of the developer is enhanced or the adhesive force between the developer and the surface of the developer holding means is enhanced to form a partially thick layer of developer which may create irregularity of the developed image.
A developing device having a member for removing toner greater in thickness than at least the toner layer brought away to the electrostatic latent image provided within a toner supply hopper in order to remove the history of development and prevent the creation of the so-called ghost image is disclosed, for example, in U.S. Pat. No. 4,100,884.
However, the device disclosed in this U.S. Patent is not of the type in which all the developer on the surface of the developer holding means is once removed as far as possible and then fresh developer is supplied to the developer holding means to effect development and therefore, it still suffers from the problems as mentioned under items (2) and (3), especially the problem (2).